1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
Optic-fiber communication systems, which have ultra-fast speed and broad band capabilities, have been practically used. An InGaAsP semiconductor laser device has been vigorously developed as a light source for use in such optic-fiber communication systems. In the InGaAsP semiconductor laser device, a substrate is made of InP, and layers are made of InGaAsP mixed crystal material which lattice-matches the InP substrate. The InGaAsP semiconductor laser device has current-light output characteristics which change greatly depending on the operating temperature. To obtain stable operation of the semiconductor laser device over a wide temperature range, a Peltier element is commonly used as a temperature control device. This results in an increase in the cost of a light module including the semiconductor laser device. To avoid the use of the temperature control device, a means for improving the temperature characteristic of the semiconductor laser device itself is desired.
The general features and characteristics of a semiconductor laser device having a multi-quantum-well structure will be briefly described. Among factors which determine the temperature characteristic of a semiconductor laser device is a so-called carrier overflow where electrons which are injected into an active layer are not confined in a well layer and then pass through the active layer. There is a known method for reducing the overflow in which the amount of light confined in the active layer is increased so that threshold-carrier density is lowered. This method has disadvantage such that the output light of the semiconductor laser device is deformed by changing the amount of the confined light.
There is another known method for reducing the carrier overflow in which the amount of confined light is not changed, but the forbidden band width of a barrier layer is increased so that the difference in the forbidden band width between the well layer and the barrier layer is increased. Although this method obtains a large band offset in the conduction band, the band offset in the valence band in also increased, resulting a reduction in hole injection efficiency.
Hereinafter, a conventional semiconductor laser device 200 will be described with reference to FIGS. 3, 4A and 4B.
FIG. 3 is a cross-sectional view illustrating the conventional semiconductor laser device 200. In FIG. 3, on an n-type InP substrate 201, an n-type InP cladding layer 202 having a thickness of 400 nm, an InGaAsP waveguide layer 203 having an energy bandgap wavelength of 1.05 xcexcm and a thickness of 50 nm, an active layer 204, an InGaAsP waveguide layer 205 having an energy bandgap wavelength of 1.05 xcexcm and a thickness of 50 nm, a p-type InP cladding layer 206 having a thickness of 400 nm, and a p-type InGaAsP contact layer 207 having a thickness of 200 nm are successively provided. An n-side electrode 210 and a p-side electrode 211 are provided on the lower side of the n-type InP substrate 201 and the upper side of the p-type InGaAsP contact layer 207, respectively.
The active layer 204 includes five InGaAsP well layers 208 having compressive strain and six InGaAsP barrier layers 209 having tensile strain which are alternately laminated. Here, the strain means an incommensurate structure between an InGaAsP layer and the n-type InP substrate 201. The degree of the strain is defined as the difference in the lattice constant. The degree of the strain is here specified by a strain factor represented by the following expressions:
(C208/209xe2x88x92C201)/C201xc3x97100(%)
where C208/209 is the lattice constant of the InGaAsP well layer 208 or the InGaAsP barrier layer 209, and C201 is the lattice constant of the n-type InP substrate 201.
The InGaAsP well layer 208 has a greater lattice constant than that of the n-type InP substrate 201, so that the strain factor of the InGaAsP well layer 208 has a positive value. The InGaAsP barrier layer 209 has a smaller lattice constant than that of the n-type InP substrate 201, so that the strain factor of the InGaAsP barrier layer 209 has a negative value.
FIG. 4A is a diagram illustrating the strain factor of each semiconductor layer in the vicinity of the active layer 204 of the conventional semiconductor laser device 200. In FIG. 4A, the six InGaAsP layers 209 (indicated by intervals A) each have the same thickness of 10 nm and the same strain factor of xe2x88x920.6%. The five InGaAsP layers 208 (indicated by intervals B) each have the same thickness of 6 nm and the same strain factor of 1.0%.
A strain amount of a layer is defined as a strain factor multiplied by a thickness of the layer. The strain amount of the whole active layer 204 is substantially zero because the positive strain amounts of the InGaAsP well layers 208 and the negative strain amounts of the InGaAsP barrier layers 209 are canceled.
The n-type InP cladding layer 202, the InGaAsP waveguide layer 203, the InGaAsP waveguide layer 205, and the p-type InP cladding layer 206 correspond to intervals C, D, E, and F, respectively, as shown in FIG. 4A.
FIG. 4B is a schematic diagram showing energy bands in the vicinity of the active layer 204. Intervals A to E indicate the respective layers of the semiconductor laser device 200, each of which corresponds to the same reference numeral in FIG. 4A. In FIG. 4B, each of the barrier layers 209 has the same energy bandgap. Band offsets X in the conduction band between the barrier layers 209 and the well layers 208 have the same value. Band offsets Y in the valence band between the barrier layers 209 and the well layers 208 have the same value. Here, a band offset is defined as the difference in an energy level between a barrier layer 209 and a well layer 208 which are adjacent to each other.
Next, the flow of electrons in the semiconductor laser device will be described. When a voltage is applied between an n-side electrode and a p-side electrode, electrons flow in the conduction band from the n-side electrode 210 to the InP substrate 201 to the n-type InP cladding layer 202 (interval C) to the InGaAsP waveguide layer 203 (interval D) to the active layer 204 (intervals A and B) to the InGaAsP waveguide layer 205 (Interval E) to the p-type InP cladding layer 206 (interval F) to the p-type InGaAsP contact layer 207 to the p-type electrode 211. At the same time, holes flow in the valence band from the p-type electrode 211 to the p-type InGaAsP contact layer 207 to the p-type InP cladding layer 206 (interval F) to the InGaAsP waveguide layer 205 (interval E) to the active layer 204 (intervals A and B) to the InGaAsP waveguide layer 203 (interval D) to the n-type InP cladding layer 202 (interval C) to the InP substrate 201 to the n-side electrode 210.
The electrons flowing in the conduction band and the holes flowing in the valence band recombine in the well layers 208 of the active layer 204, resulting in light emission.
In the conventional semiconductor laser device 200, however, the small band offset X in the conduction band between the barrier layer 209 and the well layer 208 causes electrons to overflow from the active layer 204.
Moreover, the great band offset Y in the valence band between the barrier layer 209 and the well layer 208 causes a nonuniform amount of hole injection.
One attempt to solve this problem may be made by increasing the absolute values of the strain factors of the barrier layer 209 and the well layer 208. In this case, however, the thicknesses of these layers exceed the limit of the critical thickness, so that crystal defects occur. Moreover, the thicknesses of the barrier layer 209 and the well layer 208 fluctuate, resulting in a loss in the flatness of these layers.
According to one aspect of the present invention, a semiconductor laser device includes a substrate; a p-type cladding layer and a n-type cladding layer provided on the substrate; and an active layer provided between the p-type cladding layer and the n-type cladding layer, having at least two barrier layers and at least two well layers, the at least two barrier layers and the at least well layers being disposed alternately. Band offsets in a conduction band between the at least two barrier layers and the at least two well layers are provided so as to increase from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, lattice constants of the at least two well layers are greater than a lattice constant of the substrate.
In one embodiment of this invention, lattice constants of the barrier layers are less than a lattice constant of the substrate.
In one embodiment of this invention, the substrate, the p-type cladding layer, and the n-type cladding layer are made of InP.
In one embodiment of this invention, the active layer is made of InGaAsP.
In one embodiment of this invention, lattice constants of the at least two well layers are provided so as to increase from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, lattice constants of the at least two barrier layers are provided so as to decrease from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the In molar fractions of the at least two well layers are provided to increase from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the In molar fractions of the at least two barrier layers are provided to decrease from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the at least two barrier layers each have the same energy bandgap.
In one embodiment of this invention, band offsets in a valence band between the at least two barrier layers and the at least two well layers are provided so as to decrease from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the at least two well layers have each the same energy bandgap.
In one embodiment of this invention, the sum of strain amounts of the at least two barrier layers and the at least two well layers in substantially zero.
In one embodiment of this invention, the strains between the at least two barrier layers and the at least two well layers are determined by the composition of each of the at least two barrier layers and the at least two well layers.
In one embodiment of this invention, the strain amount is represented by a strain factor multiplied by a layer thickness.
In one embodiment of this invention, the active layer is made of InGaAs.
In one embodiment of this invention, the active layer is made of InGaP.
According to another aspect of the present invention, a semiconductor laser device includes a substrate; a p-type cladding layer and a n-type cladding layer provided on the substrate; and an active layer provided between the p-type cladding layer and the n-type cladding layer, having at least two barrier layers and at least two well layers, the at least two barrier layers and the at least two well layers being disposed alternately. Band offsets in a valence band between the at least two barrier layers and the at least two well layers are provided so as to decrease from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, lattice constants of the at least two well layers are greater than a lattice constant of the substrate.
In one embodiment of this invention, lattice constants of the at least two barrier layers are smaller than a lattice constant of the substrate.
In one embodiment of this invention, the substrate, the p-type cladding layer, and the n-type cladding layer are made of InP.
In one embodiment of this invention, the active layer is made of InGaAsP.
In one embodiment of this invention, lattice constants of the at least two well layers are provided so as to increase from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, lattice constants of the at least two barrier layers are provided so as to decrease from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the In molar fractions of the at least two well layers are provided to increase from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the In molar fractions of the at least two barrier layers are provided to decrease from the n-type cladding layer side toward the p-type cladding layer side.
In one embodiment of this invention, the at least two barrier layers each have the same energy bandgap.
In one embodiment of this invention, the at least two well layers each have the same energy bandgap.
In one embodiment of this invention, the sum of strain amounts of the at least two barrier layers and the at least two well layers is substantially zero.
In one embodiment of this invention, the strains between the at least two barrier layers and the at least is two well layers are determined by the composition of each of the at least two barrier layers and the at least two well layers.
In one embodiment of this invention, the strain amount is represented by a strain factor multiplied by a layer thickness.
In one embodiment of this invention, the active layer is made of InGaAs.
In one embodiment of this invention, the active layer is made of InGaP.
In the semiconductor laser device of the present invention, the band offsets in the conduction band between the barrier layers and the well layers are gradually increased from the n-type cladding layer side toward the p-type cladding layer side. This prevents electron overflow as well as crystal defects in the active layer
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor laser device in which the strain factors of the InGaAsP well layers 208 are increased without degrading crystalline quality, and the band offsets X in the conduction band between the barrier layers 209 and the well layers 208 are increased so that the electron overflow is prevented, thereby reducing a change in the current-light output characteristics due to temperature variations.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.